1. Field of the Invention
The present invention relates to a rotation speed control system based on a throttle valve recovery position control scheme and being for use with an internal combustion engine such as a gasoline engine.
2. Description of the Prior Art
In internal combustion engines for vehicles such as automobiles, the engine is necessarily kept, for a relatively long time, in a so-called idling operation or mode in which the engine idles without contributing to delivery of power therefrom. During the idle mode, irrespective of the idle mode during warm-up immediately after starting the engine start or during normal operation following completion of the warm-up, the engine speed varies with various factors such as, for example, intake air temperature (external air temperature), cooling water temperature and oil condition, and is not always kept constant.
To follow the recent trend of imposing stringent regulations on engine-powered vehicles such as automobiles, the idling operation is to be controlled more strictly than before so as to be made compatible with the regulatory control for polluting exhaust gas and there is a need for the development of a system which can always maintain the engine at a minimum rotation speed at which a misfire is not caused, during the idling mode.
Also, contemporary innovation by electronic technology is spreading to the field of engine carburetors so that a carburetor which is controlled by an associated microcomputer, that is, a so-called electronically controlled carburetor (hereinafter referred to as ECC) may be placed on the market. With the ECC, the engine operation is controlled automatically, especially, to control the engine idle speed at a constant value. An approach to an idle speed control (hereinafter referred to as ISC) by means of the ECC is disclosed in U.S. Pat. No. 3,964,457.
With the ISC, it is possible to control the idle speed at a predetermined fixed value, without fail, which is reconciled with the stringent regulations on the engine idle speed. However, difficulties are encountered in the ISC. More particularly, according to the ISC, when the accelerator pedal of the automobile is depressed to draw the engine out of the idling mode and thereafter the accelerator pedal is released to return the engine to the idling mode, the engine operation is changed rapidly so that the engine rotation becomes unstable and a temporary reduction of the engine speed to below the idle speed occurs, resulting in engine stall, large shocks in the running of automobiles, and misfiring due to a rapid increase in the intake negative pressure which leads to polluting exhaust gas.
Further, in the ISC, it is inevitable from the viewpoint of the regulations on exhaust gases to monitor the intake negative pressure and to control the negative pressure to below a predetermined negative pressure (to above the absolute value thereof), for example, -570 mmHg. This results from the condition that, when the intake negative pressure increases to exceed a critical value (in terms of absolute value), the exhaust gas condition is degraded rapidly, thus failing to satisfy exhaust gas regulations. An approach to this problem is the provision of a throttle opener as disclosed in U.S. Pat. No. 3,266,473, for example, by which, when the intake negative pressure exceeds a predetermined value, an actuator is controlled such that the opening degree of the throttle valve is increased in preference to the controlling of the idle speed.
With the throttle opener, when the throttle valve returns to the recovery position determined by the actuator upon release of the accelerator pedal by the driver, causing a rapid decrease in the intake negative pressure which exceeds a critical negative pressure preset by the throttle opener, the throttle opener immediately responds thereto in order to control the actuator such that the intake negative pressure can be returned to the preset value. Since the actuator operates at a slow response speed, the intake negative pressure once exceeds the preset value and then approaches it, thereby greatly shortening the time interval during which the exhaust gas condition is degraded. However, since, in the course of this transitional operation of the throttle valve, the intake negative pressure fluctuates, the engine speed also fluctuates and the temporary decrease in the engine speed may cause the throttle opener to operate to increase the engine speed again, as a result, for example, when going down a slope, to car speed is temporarily increased to decrese the effect of engine braking whereby the driver experiences an uneasiness, and it causes shocks in the running of automobiles.
One measure to solve this difficulty is to ensure that the recovery or closure of the throttle valve to the idling opening is not effected rapidly but is effected gradually over a given time. Thus, the engine speed can decrease gradually to follow the throttle opening, without causing the rapid decrease in intake negative pressure. Consequently, the throttle opener will not be actuated and the transitional fluctuation in the engine speed can be prevented. Based on this principle, it may be thought to provide a mechanical damper such as a dash-pot disclosed in U.S. Pat. No. 3,081,846, for example, for the throttle valve.
However, since the essential feature of the ECC excludes the provision of a mechanical control for the carburetor as a component element thereof for the sake of pure electronic control of various functions, the provision of a mechanical element such as the dash-pot for the carburetor conflicts with the essentials for electronic control. Moreover, the provision of the mechanical element raises the cost.